The present invention relates in general to cellular communication systems, and is particularly directed to a new and improved communication control mechanism for controlling the hand-off of frequency channels through which communications are conducted between base stations of adjacent cells and a mobile transceiver as the mobile transceiver moves between those cells.
Wireless (cellular) communication service providers customarily supply wireless communication capability to (mobile) subscribers located within a geographic area, through the use of a relatively limited number of communication channels. In order to optimize coverage within the geographical area of interest, the service provider typically subdivides the area into a cluster or multiple clusters of base stations. In addition, in order to minimize interference from adjacent or nearby cells, the service provider may employ some form of frequency reallocation (or reuse) scheme, such as that described in the U.S. Pat. No. 4,144,496, as a non-limiting example.
In such a spatially distributed or xe2x80x98clusterxe2x80x99 network architecture, a fixed number of sectors (i) are served by a cluster of (k) base stations. This has the effect of subdividing the number of available channels N by the product of i and k, namely by (i*k). Unfortunately, with today""s expanding traffic, particularly in densely populated urban areas, service providers face the eventuality of running out of channels to meet demand.
One solution is to construct more base stations and reduce power levelsxe2x80x94which is both hardware intensive and expensive. Another scheme is to reuse channels in time (TDMA) or in frequency (CDMA). Other approaches, such as described in the above-referenced patent, include dynamic allocation of frequencies or channels to accommodate channel demand. Initially, the relatively poor efficiency of frequency allocation schemes was not a significant problem as the demand was small and the number of available channels was more than adequate. However, as demand increased, new channel assignment and frequency reuse strategies were developed.
Such schemes have included sectorization of cells to minimize interference, and dynamic allocation or xe2x80x98borrowingxe2x80x99 of channels from other cells with a cluster, to meet unbalanced demand within the cluster. A new and promising approach is to spatially separate channels using switched or steered antenna beams. The overall objective of any strategy is to maximize the number of channels available, subject to an acceptable carrier (C) to interference (I) ratio, with the current industry standard being a figure of merit (or C/I ratio) of 18 dB.
Sectorization is a technique that uses fixed beams formed by directional antenna (phased) arrays installed at the base stations to divide the cell into an integral number of smaller cells. This technique serves to reduce interference to the base station, by attenuating channel interference to those mobile subscribers who are not located in that sector""s beam. It also reduces interference to the mobile subscriber, by attenuating channel interference from base stations transmitting in a direction that is predominately away from the location of the mobile subscriber. However, as the number of sectors increases, the number of channels per sector necessarily decreases, thereby reducing the figure of merit. Ideally, at the time of system installation, there would be no sectorization, which would greatly increase system capacity.
Regardless of the channel allocation mechanism employed, whenever a mobile subscriber moves from one cell to another, it is necessary to change the frequency channel used to conduct communications with the base station in the xe2x80x98oldxe2x80x99 cell from which the mobile transceiver is departing to a new frequency channel used to conduct communications with the base station in the xe2x80x98newxe2x80x99 cell which the mobile transceiver is entering.
Techniques using steered beam antennas have unique problems accomplishing this handoff between cells. In particular, the xe2x80x98newxe2x80x99 cell has the problem of where to point its narrowbeam antenna. The mobile subscriber is waiting on transmission from the new base station to transmit on the new frequency. If the new base station points the beam in the wrong direction, then the mobile subscriber sees no signal, does not synchronize and does not transmit. After the elapse of a prescribed period of time with no communication, the call will be dropped. The problem then is for the new base station to determine the correct beam to the mobile subscriber.
One mechanism for performing such frequency channel reuse/reallocation (or hand-off from the previous base station to the new base station) is described in the U.S. patent to Forssen et al, U.S. Pat. No. 5,615,409. This scheme involves the base station using an xe2x80x98intermediatexe2x80x99 channel to determine the direction of the mobile transceiver relative to it. It then assigns the mobile transceiver to an available narrowbeam channel. Because this technique requires what could otherwise be used for a regular communication channel be employed as an intermediate construct channel to determine the direction of the mobile transceiver, it necessarily reduces the number of available precious resources (channels).
In accordance with the present invention, this drawback is effectively obviated by a channel hand-off communication control mechanism that uses the very channels that are employed for communications between base stations of adjacent cells and a mobile transceiver as the mobile transceiver moves between those cells, to locate the mobile transceiver relative to the base stations, so that the acquiring base station may readily place a narrowbeam channel on the mobile transceiver at hand-off. Each base station employs a phased array antenna, which allows the base station to controllably define its antenna coverage pattern with respect to any mobile transceiver, so as to minimize interference from one or more other transceivers, and thereby reduce frequency reuse distance.
Pursuant to a first embodiment of the invention, when the quality of a narrowbeam link between the mobile subscriber and an already acquired cell base station indicates that the mobile transceiver is approaching a cell boundary with a new cell, the already acquired base station will initiate a hand-off sequence with the acquiring base station in the new cell to which the mobile subscriber is moving. For this purpose, the current base station will forward a message to the new base station that a channel hand-off is to commence. This hand-off initiating message will contain the identification of the communication channel currently employed by the mobile transceiver.
In response to this message, the acquiring base station employs one of the antenna elements of its phased array antenna to transmit an omnidirectional burst on a new communication channel to which mobile transceiver is to tune itself for conducting communications with the acquiring base station at hand-off, when the mobile transceiver enters cell. In response this burst signal on the new channel, the mobile transceiver transmits a reply signal on the new channel, which is processed by the acquiring base station to derive a steering vector representative of the direction of the mobile transceiver relative to the new base station.
The new base station employs this derived steering vector to adjust the directivity pattern of its phased array antenna, so as to place a narrowbeam pattern of the new communication channel in the direction of mobile transceiver, completing the hand-off. The new base station proceeds to conduct narrow beam communications with the mobile transceiver on the new communication channel. Using its ability to control the directivity of the narrowbeam lobe by way of its phased array antenna, the new base station continues to communicate with and track the mobile transceiver as long as the mobile transceiver is located in the new cell.
In a second embodiment of the invention, the acquiring base station a xe2x80x98sectorizedxe2x80x99 burst in place of an omni burst of the first embodiment to locate the mobile subscriber. This sectorized burst is confined to a prescribed spatial sector sourced from the new base station toward the current cell in which the mobile subscriber is currently located.
In a third embodiment of the invention, the base station of the current cell from which the mobile transceiver is about to depart into the new cell determines the direction of the mobile transceiver relative to the already acquired base station, and generates a first steering vector associated with this direction. This steering vector is conveyed as part of the hand-off initiating message to the new or acquiring base station. In response to this first steering vector message, the new base station generates a second steering vector, representative of the direction of the mobile transceiver relative to that base station for the new communication frequency channel to be used between the mobile transceiver and the new base station at channel hand-off. Using this second steering vector, the new base station adjusts the directivity pattern of its phased array antenna, so as to place a narrowbeam pattern of the new communication channel in the direction of the mobile transceiver.
When the mobile subscriber responds on the new channel, hand-off is complete between the base stations, and the mobile transceiver thereafter communicates with the second base station as it enters into and travels through the new cell. The new base station then proceeds to conduct narrow beam communications with the mobile transceiver on the new communication channel, using its phased array antenna to track and communicate with the mobile transceiver as long as the mobile transceiver is located in the new cell.
In a fourth embodiment of the invention, the new or acquiring base station xe2x80x98pretunesxe2x80x99 its transceiver to the xe2x80x98oldxe2x80x99 or xe2x80x98pre hand-offxe2x80x99 frequency employed by the current or already acquired base station, in order to determine the direction of the mobile subscriber, prior to channel hand-off. In response to a pre hand-off message, the new base station uses its phased-array antenna to place a narrowbeam pattern for the current channel being employed by the already acquired base station in the general direction of the cell in which the mobile subscriber is currently located.
The new base station then monitors the current channel to derive a steering vector representative of the direction of the mobile transceiver relative to the new base station. At hand-off, the new base station employs the steering vector derived for the previous channel to place a narrowbeam lobe for the new channel in the direction of the mobile subscriber. When the mobile subscriber responds on the new narrowbeam channel, hand-off is complete between the base stations, and the mobile transceiver thereafter communicates with the new base station as it enters into and travels through new cell.